Mesh Motion (Piston valve)
 

Hallo to all,

I am using CFX for simulating pressure wave propagation created by restricting flowrate by movement of piston (20 mm in 0.03 sec, linear relationship, piston speed is constant) inside pipe.The fluid water, taking into account the compressiblity by using Material group IAPWS IF97, turbulent model SST, mesh motion is specified location.

During the simulation, I do not get any error message and the piston moves also 20 mm without any negative volume. However, with smaller time step, I get unrealistic results. For example, for time step 0.01 sec, the maximum pressure rise is 0.2 bar which is corresponding maximum movement (20 mm),but for smaller time step for example (0.0005 sec or smaller), there is an unrealistic pressure increasment for the first few time step (for the first time step (0.0005 sec correspondes to 0.33 mm movement), pressure rise is 1 bar or even more with smaller time step). After 3 to 5 time steps, the pressure decreases again and then lineraly increases till 0.03 sec. Again, there is another fluctuation or oscillating in the pressure, when the piston moves back. I refined the mesh and using different turbulent models, but there was no improvement. Could someone help, any ideas or advice, thanks in advance.